Indicated airspeed (IAS) is the airspeed read directly from the airspeed indicator on an aircraft, driven by the pitot-static system. The IAS varies considerably from the true airspeed depending upon atmospheric conditions and configuration of the aircraft and even upon how the aircraft is being flown. Calibrated airspeed (CAS) is the IAS corrected for instrument and installation errors at a standard air pressure and temperature, the International Standard Atmosphere.
An aircraft's indicated airspeed in knots is typically abbreviated KIAS for "Knots-Indicated Air Speed" (vs. KCAS for calibrated airspeed and KTAS for true airspeed).
The IAS is an important value for the pilot because it is the indicated speeds which are specified in the aircraft pilot's manual for such important values as the stall speed and various airframe structurally limited speeds. These speeds, in true airspeed terms, vary considerably depending upon density altitude; but the aircraft will always stall at the same indicated airspeed for a configuration, regardless of density altitude.
Furthermore the IAS is specified in some regulations and by air traffic control to pilots since the airspeed indicator displays that speed (by definition) and it is the pilot's primary airspeed reference in an aircraft when operated below transonic or supersonic speeds.
Read more about Indicated Airspeed: IAS Vs CAS, IAS and V Speeds, IAS and Navigation, Other Airspeeds